The GMC CCKW

Hello all, can anyone out there explain the differences between the 270 and 302 engines, with regards to, pistons camshaft, cylinder head, is the 302 block the same as the 270 but just larger bores ?
Are the 270 pistons steel ?
Why can the 302 rev higher than the 270 ?

Rich UK

Go back to the home page:

http://cckw.org/270_to_302.htm

and read my description of the differences of the many different 270 and 302 models.

I have had a 302 in my CCKW for 1000's of miles now with no regrets (except I have the Stromberg carbed engine vs the Holley one) and love it! I would never go back as I tow trailers, heavy ones or guns most of the time.

Steve

Thanks Dr Deuce, I have read that but was more interested in the internal differences of the 2 engines like Piston dia and stroke, cam timing, type of differences in the cylinder heads ect.

The other thing is, can I ask your opinion on tyre sizes on the rears of GMC trucks I am aware they should be matched sizes but how matched should they be?
I assumed that as all the rear tyres on my truck are made by Uniroyal they would be the same size, well how wrong I was, they differ be up to 1 1/2 inches on the circumference !!!!!!!!!!! is this tooo much. What is your opinion of dropping off the short prop between the very rear axle and the pillow block would this overload the one driving axle ? Would not be towing only loaded with camping equipment etc.

R Cubed

Forget taking out the short shaft. You will get stuck constantly. Been there. Even Annie the crane with the rear rear blown and that tremendous weight of the crane right over the rears gets stuck some times!

The tires need to be very closely matched. As a rule of thumb: on a flat, level, paved surface with warmed tires going straight ahead, you should be able to push the truck by yourself in neutral!!!! If you cannot, something is amiss or binding.

The 50's 270 and 302 have flat top pistons. The WW2 engines; 270 and smaller had "turbo-top". The heads were changed to match among other things. All 270's had a 3 25/32 inch bore as I recall and the 302 had a 4 inch bore. All had a 4 inch stroke. I "think" the cams are all the same but am not sure. I am pretty sure that the 50's engines, all of them have the same cam; mil and all civy versions.

Hope this helps

That's a bit more of the info I was after, I have heard that the later 270 were producing similar HP figures to the 302 ? about 130 HP what was the differences in the early and late 270's then !!!!!!!

If you could help do you know of any hotrodding places which might have more information on this subject.

Thanks Rich

The later 270's have bigger intake ports and intake manifold (mabe valves to, not sure w/o looking it up). They also have sheetmetal rockerarms instead of cast iron. The carb had a larger dia opening too so it breathes better. You are correct about the 130 or maybe it was 145 hp for the later 270. The 302 is 160. The 302 can go up to 339 cid with boring. I am not sure if the 302 manifold will fit the 270 (50's) as it has a 2 bbl carb.

There are hot rodding books for the Chev and GMC engines. The GMC engines from the 50's were favorites in some circles. Of course, the usual polishing of the ports is an easy thing to do and helps quite a bit.

Hope this helps

Sounds like you are full of useful information, do you know if the military 270's can be bored much bigger ?
Was there any point when the 270 or 302 heads went to 12 ports, rather than the 7 on a standard military head ?
I think I will look out for some of those books, only want to tune up my truck a bit to help on the hills maybe a different cam might produce more torque, or perhaps a bit more HP would help overall, we shall what happens.

Thanks again
Rich

I don't understand your 12 port vs 7 port?

I suspect that the 302 has different sand casting to allow the bigger bore. The 270 had that because the 256 was maxed out.

Over here, I find 302's all over the place. I have 4 of them right now and have found 2 more which I will buy this year if possible. That is in addition to the one 50's 270 that is in my tanker already.

Sorry must be the English terms, 12 ports refers to the number of inlet and exhaust openings on the side of the cylinder head where the inlet manifold and exhaust manifold bolts to, some of the inlet ports and exhaust ports are shared, thus 7 ports on a standard head. Were any of these made with one exhaust and one inlet port per cylinder ? so multiply this by 6 cylinders and you have 12 ports, 6 inlet and 6 exhaust, this would create very easy breathing for the engine !!!!!!!!

Not by GMC to my knowledge